Take this juice for free

Near my office in Shanghai there’s a small shop where you can find western products with discounts. It’s interesting how the woman that works there attracts customers. A lot of people go there because her.
I remember few months ago, one of the first time I went there to buy something. She was nice, always trying to speak you in English. And when I was going to left the shop she said, hey take this juice for you, I give it to you for free.
Everybody thinks she’s nice, but thanks to that gesture, she attracts more customers.
It’s a basic Cialdini’s principles from his book Influence. Reciprocity. Basically it means that people feel the urge to repay to someone’s kindness. Which translated into this case, it means people will come back to that shop just because the gift.
A small gesture can make a big difference.
Even knowing this influence, at the end, you continue buying stuff there.